Ferrites are important materials to industry because of their wide use as magnetic materials and as catalysts. The synthesis of submicron sized ferrite powders has generally received particular attention due to a number of factors including higher green densities and improved electric, magnetic and surface properties. Low temperature syntheses of such materials have been found to generally result in finer particles but have also been the focus of much interest because of unique or unusual particle forms and chemical stoichiometry considerations.
Traditional techniques for the preparation of fine particles of ferrites and ceramic materials in general have involved repeated firings of the component oxides, hydroxides, or carbonates at high temperatures with frequent grindings and mixings. The result is usually crystalline but inhomogeneous materials having low surface area. To induce better diffusion and homogeneity, chemical precursor techniques such as, for example, electrolytic coprecipitation, coprecipitation of oxalates, spray drying of mixed sulfate solutions, and thermal decomposition of mixed metal acetate complexes have been known in the art. Two reviews have been published on the topic: R. Lal and P. Ramakrishman, Trans. Indian Cer. Soc. 38, 166, 1979, and B. K. Das, Preparation and Characterization of Materials, Honig & Rao, Eds., Academic Press, 75, 1981. The advantage in using precursor techniques has been the production of homogeneous, well mixed oxides at reduced formation temperatures. However, even though the reported synthesis temperatures for these techniques are lower than those required in conventional ceramic syntheses, there is positively a need in the art to lower synthesis temperatures further.
A typical chemical precursor, such as ferric oxalate or ferric nitrate, is an oxidizer or has an oxygen-containing anion and can be decomposed endothermically to produce an oxide residue, such as .gamma.Fe.sub.2 O.sub.3. It is well known that the time required for such decompositions can be considerably shortened if the precursor is heated in the presence of a reducing agent, such as hydrazine. The metal salts are normally reduced in solution and yield metal powders.
A relevant chemical precursor method which has recently been reported involves the preparation of ferromagnetic spinels, such as MnFe.sub.2 O.sub.4, CoFe.sub.2 O.sub.4 and NiFe.sub.2 O.sub.4, by the thermal decomposition of mixtures of pyridine and pyridine-1-oxide complexes of the corresponding metal nitrates. See R. T. Richardson, J. of Mat. Sc. 15, 2569, 1980. Using this technique, amorphous oxides in a glassy state are formed between 300.degree. and 400.degree. C. and crystallization takes place at temperatures between 800.degree. and 1000.degree. C. Due to the use of the organic pyridine, however, there is a real possibility of carbon contamination in the ferrite products.
The applicant herein has prepared and decomposed mixed metal oxalate hydrazinate precursors at temperatures as low as 120.degree. to 200.degree. C. to yield oxide spinels such as ferrites and cobaltites. See K. C. Patil, D. Gajapathy, V. R. Pai Verneker, Mater. Res. Bull. 17, 29, 1982, and J. of Mat. Sci. Letters 2, 272, 1983. The approach for these syntheses is the incorporation of an inorganic reducing fuel molecule, such as hydrazine, within the metal salt molecules yielding precursor molecules which are capable of rapid decomposition in air to the corresponding oxides.
A typical synthesis begins with the preparation of the precursor by dissolving the oxalate or oxalates (single or double salt synthesis, respectively) into excess liquid hydrazine hydrate according to the following formula: EQU MC.sub.2 O.sub.4.2H.sub.2 O+N.sub.2 H.sub.4.H.sub.2 O.fwdarw.MC.sub.2 O.sub.4.xN.sub.2 H.sub.4 +H.sub.2 O
where M may be a metal such as manganese, iron, nickel, cobalt, titanium, cadmium, magnesium, aluminum, boron, etc. and x is an integer dependent on the valency of the metal. Alternatively, the oxalate hydrazinate precursor may be prepared from one or more metal powders and ammonium oxalate as follows: EQU M+(NH.sub.4).sub.2 C.sub.2 O.sub.4.H.sub.2 O+xN.sub.2 H.sub.4.H.sub.2 O.fwdarw.MC.sub.2 O.sub.4.xN.sub.2 H.sub.4 +H.sub.2 O+H.sub.2 +2NH.sub.3
where M and x are the same as above. The precursor is thereafter recovered, placed in a reactor and heated to a temperature up to 200.degree. C. in air for 30 to 90 minutes to decompose the oxalate hydrazinate. Thus, for example, if ferric oxalate is converted into ferric oxalate hydrazinate, a coordination compound of oxalate and hydrazine, this precursor decomposes exothermically at as low a temperature as 120.degree. C. producing fine amorphous .gamma.Fe.sub.2 O.sub.3 particles which are highly magnetic and capable of high green density. Crystallization of these particles takes place at around 340.degree. C. Ultrafine particles of iron may be produced if the decomposition is carried out under nitrogen, hydrogen or vacuum conditions. Similarly, double precursor salts, such as MgFe.sub.2 (C.sub.2 O.sub.4).sub.3.5N.sub.2 H.sub.4, produce ceramic materials such as MgFe.sub.2 O.sub.4 on heating in air and alloys such as MgFe.sub.2 on heating under hydrogen or vacuum conditions.